Pop-up menus based on text properties

This is a library for computing context menus based on text properties and overlays. The intended use is to have tools that annotate source code and others that use these annotations, without requiring a direct coupling between them, but maintaining discoverability.

Major modes that wish to use this library should first define an appropriate value for prop-menu-item-functions. Then, they should bind prop-menu-by-completing-read to an appropriate key. Optionally, a mouse pop-up can be added by binding prop-menu-show-menu to a mouse event.

For example, the following value for prop-menu-item-functions creates a popup menu that will describe faces that are set in either text or overlay properties:

(setq-local prop-menu-item-functions
            (list (lambda (plist)
                    (let ((face (plist-get plist 'face)))
                      (when face
                        (list (list "Describe face" (lambda ()
                                                      (interactive)
                                                      (describe-face face)))))))))

Note that this setting requires lexical scope.